This invention relates to improvements in electrical power assisted steering systems.
In a typical electric power assisted steering system, an electric motor, such as a three phase DC electric motor, is connected to a part of the steering mechanism, typically to the steering shaft that connects the steering wheel of the vehicle to the road wheels. A sensor, such as a torque sensor, produces a signal indicative of the torque applied to the steering wheel by the driver, and this signal is fed into a microprocessor. The microprocessor uses this signal to produce control signals for the motor which are indicative of the torque or current that is required from the motor. These control signals are converted into voltage waveforms for each phase of the motor within the microprocessor, and these in turn are transmitted from the microprocessor to a motor bridge driver.
The motor bridge driver converts the control signals, which are typically low level voltage waveforms, into higher level voltage drive signals that are applied to the respective phases of a motor bridge. A typical bridge comprises a set of switches that selectively apply current from a supply to the phases of the motor as a function of the high level voltage drive signals applied to the switches from the bridge driver circuit. By controlling the switches the current in the motor can be controlled relative to the motor rotor position, allowing the torque produced by the motor to be controlled. The motor in use is thereby caused to apply an assistance torque to the steering system that helps, or assists, the driver in turning of the steering wheel. Because this torque affects the output of the torque sensor, this forms a type of closed loop control allowing accurate control of the motor torque to be achieved.
The torque sensor typically comprises a torsion bar and two angular position sensors, one of which provides an output signal representing the angular position of the steering system on one side of the torsion bar and the other an output signal representing the angular position of the steering system on the other side of the torsion bar. When no torque is applied, the two output signals will be in alignment, but as a torque is applied the torsion bar twists causing the two angular position sensors to move out of alignment. This relative change in the output signals provides the measurement of torque needed.
To provide additional margin of safety in the event of a fault it is common to use a dual channel torque sensor, which produces two channels of information that each respectively provide a torque measurement. In use, the torque indicated by each channel is checked against the other and if they are in agreement it can be assumed that the torque value is reliable. If they are not in agreement, one or both channels may be faulty and an error flag can be raised. Typically when this happens the assistance torque is not applied by the motor.
Although dual channel torque sensors give increased safety it is not possible to continue to safely apply assistance torque if one channel is faulty even if the other is not, partly because it may not be possible to tell which channel is faulty and which is reliable, and also because there is no way to provide protection against a subsequent fault occurring in the one remaining good channel.